The present invention relates to a method and system for monitoring the growth of cracks liable to occur in loaded structures.
It is known that, in structures undergoing strong stresses such as aircrafts, ships, vehicles, platforms and pipes structures, cracks are liable to appear and/or grow in certain critical points. For example, such critical points correspond to the position of rivets joining up metal plates constituting the skin of the fuselage of an aircraft, in which plates risks of cracks may occur around the edges of the holes receiving said rivets.
For obvious reasons of security, said critical points need to be inspected periodically, in order to check that any cracks occurring do not affect too much the mechanical strength of the structure. But for reasons of economy, maintenance should be adapted to the use of the structure and inspections and repairs should only be carried out if a damage of sufficient importance is expected to have occurred. Indeed, in composite or metallic parts which are subjected to a complex stress system when in use, damages due to fatigue can appear at very variable time, depending on how the said parts have been used.
In order to follow the propagation of cracks, it is already known to use, especially in laboratory tests, devices such as:
rip links, namely wires or fiber glass yarns which are bonded as close as possible to the expected damage, perpendicularly to the crack; these are wires which break as the crack gradually grows.
indestructible control sensors such as:
supersonic, eddy current or other type of sensors which are placed close to the critical points. As soon as the fault reaches the detectability level, namely the size which can be detected by the method, said fault is identified by the device; PA2 sound, electric or optical probes for determining the appearance of a fatigue crack. PA2 (1.1) the variation dK=(K/C).multidot.dC PA2 (1.2) the theoretical speed dl/dN of crack growth as a function of the number N of cycles of stress variations with the help of an expression dl/dN=F(R,dK) in which F(R,dK) is a function of the ratio R=m/M and of the variation dK of the stress intensity factor; PA2 (1.3) by integrating the theoretical speed dl/dN, the theoretical increase of the length of the crack; PA2 (1.4) by adding said length determined by the corresponding operation (b) and said theoretical increase obtained under (1.3), the instant theoretical length of the crack.
Such devices are generally expensive and can only perform a good monitoring operation from a very limited and accessible zone. They also require a very particular maintenance.